Nature of the excited states ofHe2

The low-lying excited states of ${\mathrm{He}}_2$ have been examined using spatially projected generalized-valence-bond wave functions. Typically the excited-state potential curves are repulsive at large $R$ (internuclear distance) but exhibit an attractive well at small $R$. We find that the repulsion at large $R$ results from an unfavorable exchange interaction of a Rydberg orbital mostly on one He with the two core orbitals on the other He. This arises from the restrictions on the molecular wave function implicit in the Pauli principle and is similar in nature to the repulsive pair-pair interactions between ground state He atoms. For $R<\mathrm{}3\mathrm{}{a}_0$ the Rydberg orbital is large compared with $R$ and the shape of the potential curve is determined by core-core interactions. For the $A{}_{}{}^1{}_{}{}^{}{\Sigma }_u_{}^{+}{}_{}{}^{}$ state, the calculated maximum in the potential curve is 0.0607 eV at 3.09 Å, in excellent agreement with experimentally derived values. Results are presented for several other excited states.